

:: Section 8Curvilinear Motion and Stokes Number
II. Stokes NumberCurvilinear motion is characterized by a dimensionless number called the Stokes number (Stk). It is the ratio of the stopping distance of a particle to a characteristic dimension of the obstacle. For example, for a flow of diameter d_{c} making a 90° turn, perpendicular to a cylinder the Stokes number is where U is the flow velocity, for Re_{p} < 1.0. When Stk >> 1, particles continue moving in a straight line when the gas turns; when Stk << 1, particles follow the gas streamlines perfectly and make the turn. So Stokes number is the ratio of a particle's "persistence" to the size of the obstacle and it is used to characterize inertial impaction, which is described in the next section.
